Cherry-pick #23229 to 7.x: [Heartbeat] Enable script processor

[andrewvc]

Cherry-pick of PR #23229 to 7.x branch. Original message:

Enables the script processor for heartbeat. This is an opt-in processor, but there's really never been a concrete need for that. This fixes #22788 .

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

How to test this PR locally

Add a script section to the processors block, it will work!

Use cases

Mostly useful for situations where people want to apply novel complex logic. See #22788

[elasticmachine]

Pinging @elastic/uptime (Team:Uptime)

[blakerouse]

Backport looks good.

[elasticmachine]

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Build stats

• Build Cause: Pull request #23768 opened

• • Start Time: 2021-01-29T21:35:15.821+0000

• Duration: 30 min 32 sec

• Commit: 27678dd

Test stats 🧪

Test	Results
Failed	0
Passed	3144
Skipped	76
Total	3220

💚 Flaky test report

Tests succeeded.

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Test stats 🧪

Test	Results
Failed	0
Passed	3144
Skipped	76
Total	3220